1. Field of the Invention
The invention relates to a vehicle control unit that executes uphill climbing control while a vehicle is traveling on an uphill slope.
2. Description of Related Art
Japanese Patent Application Publication No. 2001-225657 describes a drive power distribution control unit for a four-wheel-drive vehicle, which suppresses occurrence of a phenomenon of tight corner braking upon acceleration during forward turning or upon deceleration during backward tuning on a flat road, and which improves the starting performance at the time of an uphill start on, for example, an uphill slope with a low friction coefficient (μ).
Whether to carry out uphill start control is determined depending upon whether the condition set based on, for example, a vehicle speed and an uphill gradient continues to be satisfied for a set time. Because a drive power distribution ratio for the four-wheel-drive vehicle is determined based on an uphill gradient, it is desirable to calculate an uphill gradient at the time of an uphill start in order to improve the starting performance. In a case where the vehicle stops after travelling at a low speed on an uphill slope, the uphill gradient when the vehicle stops may differ from the uphill gradient at the start of the uphill start control. Accordingly, the uphill start control that has been executed until the vehicle stops is cancelled, and then it is determined again whether the condition for uphill start control is satisfied. After that, the uphill start control is executed again.
However, when the vehicle starts on an uphill slope, the vehicle may start ascending the uphill slope after descending slightly, depending on a pedal operation or a shift operation. In this case, the uphill gradient may differ and the uphill start control is cancelled. In order to execute the uphill start control again in this case, the set time used to determine whether to carry out uphill start control may be shortened. However, if the set time is shortened, there is a possibility that a condition similar to that for the uphill start control will be satisfied upon acceleration from standstill on a flat road. Therefore, if the set time is excessively shortened, the uphill start control is executed upon acceleration from standstill on a flat road, which gives an occupant a sense of discomfort.